1. Field of the Invention
The present invention relates to an electron beam writing apparatus equipped with means for correcting an image placement error to write each pattern and an image placement error correcting method thereof, and particularly to an electron beam writing apparatus and an image placement error correcting method using an EUV mask.
2. Background Art
EUV lithography is now cited as a lithography technique useful as an alternative to photolithography. Since a reflection type mask (hereinafter called “EUV mask”) is used in the EUV lithography, an electrostatic chuck for holding a mask from its back surface is used.
While the flatness of a reflective surface of a substrate, i.e., its surface exerts a direct influence on the accuracy of position of each pattern in the EUV mask, it is important to consider the relationship between the shape of the back surface of the EUV mask and the chuck surface (front face) of the electrostatic chuck for the purpose of maintaining satisfactory flatness of such a surface. This is because when foreign substances such as particles are intruded between the back surface of the EUV mask and the surface of the electrostatic chuck, the satisfactory flatness of the EUV mask surface is inhibited due to the fact that the electrostatic chuck is not brought into close contact with the EUV mask locally at its intruded location. This is also because each pattern written onto the surface on a consequential basis is distorted with deformation of the surface shape by correction of the back surface shape of the EUV mask by the electrostatic chuck.
Therefore, there has been a need to correct the image placement error of the pattern for the purpose of improving image placement accuracy of the EUV mask held by the electrostatic chuck. With the foregoing in view, it has heretofore been practiced to measure the back surface shape of the EUV mask by an optical measuring apparatus, bring back surface shape data obtained by this measurement and back shape data corrected by the electrostatic chuck to approximations using tertiary or more polynomial expressions respectively and determine a correction amount relative to the image placement error from the resultant approximate expressions (refer to, for example, a patent document 1).
Meanwhile, a pin chuck is used as the electrostatic chuck to reduce an influence exerted on the flatness due to the intrusion of the foreign substances or the like between the back surface of the EUV mask and the surface of the electrostatic chuck upon its chuck.
Since the back surface shape data of the EUV mask measured by the optical measuring apparatus has the smaller pixel size than the pitch of the pin chuck, the shape of a back surface of a position brought into no contact with each pin is also measured between the pitch of the pin chuck where the EUV mask is held by the pin chuck from its back surface.
FIG. 3 is a partly enlarged view of an EUV mask held by a pin chuck. FIG. 3 shows a state in which when the EUV mask M is held by the pin chuck C from its back surface where a protruded shape i exists in the back surface of the EUV mask, the protruded shape i intrudes into the pitch of the pin chuck C between pins p1 and p2 thereof adjacent to each other without making contact with both of the pins p1 and p2.
There was influenced that when a correction amount relative to an image placement error was determined using the above polynomial expressions with respect to the measured back surface shape data and the corrected back surface data as in the related art where such a state as shown in FIG. 3 has occurred, an error occurred due to the difference in resolution, so that an accurate correction amount could not be obtained, thus resulting in degradation of image placement accuracy.
Incidentally, the shape and pitch of the pin chuck are not defined in terms of standards unified under the SEMI standard. There is a possibility that they will differ, for every EUV scanner or for every customer who takes its delivery.
With the foregoing problems in view, the present invention therefore aims to provide an image placement error correcting method capable of calculating a high-accuracy correction amount relative to an image placement error in consideration of a difference in resolution between the shape of a back surface of an EUV mask and the shape of a surface of a pin chuck, and an electron beam writing apparatus equipped with image placement error correcting means.    [Patent Document 1] Japanese Patent Application Laid-Open No.